Why is my phone stuck in a loop when turning on?

A few months ago, one day my ZTE Obsidian didn't connect to the wifi, and it stayed like that for a month or two, eating up my data. I was going to send it in to get it fixed, when it did a software update. The software update turned off my phone and sent it in a loop where it would load up until it started optimizing the apps, this is where it would stop and turn back off and repeat itself. I want to know why it does this and how I can send it either fix it and/or send it in again. I sent it in once before and it didn't solve the problem, it did another update and the whole thing started doing the loop again.